High pressure radical polymerisation is typically operated at high pressures up to 4000 bar. In known high pressure reactor systems the starting monomer(s) need to be compressed (pressurised) before introduced to the actual high pressure radical polymerisation reactor. Compressor lubricants are conventionally used in the hyper compressor(s) for cylinder lubrication to enable the mechanically demanding compression step of starting monomer(s). It is well known that small amounts of the lubricant normally leaks through the seals into the reactor and mixes with the monomer(s). In consequence the reaction mixture contains traces (up to hundreds of ppm, typically 300 ppm) of the compressor lubricant during the actual polymerisation step of the monomer(s). These traces of compressor lubricants can have an effect on the final polymer performance.
Compressor lubricant means herein a lubricant used in compressor(s), i.e. in hyper compressor(s), for cylinder lubrication.
Conventional commercial hyper compressor lubricants e.g. polyalkylene glycol (PAG):R—[CxRyHz—O]n—H,wherein R can be H or straight chain or branched hydrocarbyl and x, y, z, n are independent integers that can vary in a known manner, and lubricants based on a mineral oil (by-product in the distillation of petroleum) can be mentioned. Hyper compressor lubricants which are based on mineral oils that meet the requirements set for the white mineral oil in European Directive 2002/72/EC, Annex V, for plastics used in food contact, are used e.g. for polymerising polymers especially for the food and pharmaceutical industry. Such mineral oil-based lubricants contain usually lubricant additive(s) and may also contain other type of additive(s), such as antioxidants.
WO2009012041 of Dow discloses a high pressure polymerisation process, wherein compressors are used for pressurising the reactants, i.e. one or more monomer(s), the compressor lubricant may have an effect on the properties of the polymerised polymer. The document describes the use of a polyol polyether which comprises one or none hydroxyl functionality as a compressor lubricant for preventing premature crosslinking particularly of silane modified HP polyolefines. This is due to the presence of both multiple hydroxyl groups and the hydrophilic ethylene oxide groups, these lubricants are quite hydrophilic, and this can result in increased water uptake by the polymer, especially a silane modified polymer. This is contrary to the present invention that uses mineral oils that are not affecting the crosslinking process.
WO2009012092 of Dow discloses a composition that comprise a high pressure polyolefin free of silane functionality and a hydrophobic polyether polyol of PAG type wherein at least 50% of its molecules comprise no more than a single hydroxyl functionality. The component appears to originate from a compressor lubricant. The composition is for cable applications and is stated to reduce electrical losses in medium and high voltage power cables.
EP2499176, EP2499197 & EP2499175 of Borealis disclose a polymer composition with improved electrical properties. This is achieved by using mineral oil in the hyper compressor to decrease conductivity of the insulation layer in a cable by optimising the space charge by controlling the overall polarity of the polymer. The object of the invention is to reduce any polarity of the polymer in order to get good electrical properties for medium and high voltage power cables.
There is a continuous need in the field of silane crosslinkable polymers which are suitable for demanding polymer applications such as wire and cable applications with high requirements and stringent regulations.